Porous assembly of an antifungal protein mediated by zinc and sulfonato-calix[8]arene

Controlled protein assembly holds great potential in the fabrication of biohybrid materials. However, the structural diversity and complexity of proteins present an obstacle to controlled assembly. Supramolecular chemistry is a possible solution as it offers tools to mediate self-assembly with molecular precision. This paper deals with the calixarene- and zinc-mediated assembly and crystallization of the histidine-rich Penicillium chrysogenum antifungal protein B (PAFB). We report crystal structures of pure PAFB, PAFB in complex with Zn²⁺, and the ternary complex of PAFB, Zn²⁺ and sulfonato-calix[8]arene (sclx₈). A comparison of the three crystal structures revealed the structural plasticity of PAFB. While the flexible and highly anionic sclx₈ resulted in large molecular weight aggregates of PAFB in solution, diffraction-quality crystals of PAFB-sclx₈ were not obtained. We report crystals of PAFB-Zn²⁺-sclx₈ in which a trinuclear zinc cluster occurred adjacent to a calixarene binding site. Interestingly, the combination of sclx₈ complexation and zinc coordination resulted in a porous framework with channels of circa 2 nm diameter. Detailed analysis of the crystal structure highlighted novel molecular recognition features. This research enriches the set of supramolecular interactions available to promote protein assembly.